Heat treatment of aluminum alloys



United States Patent HEAT TREATMENT OF ALUMINUM ALLOYS Otto Schaaber, Schorndorf, Germany Al-Mg-Si type as long as they do not contain substantial quantities of copper. The amount of copper suppressing the detrimental efiect of room temperature storage varies but is to a certain degree proportional to the amount 01 5 MgzSi present. No Drawing. Application May 6, 1949, It is an object of this invention to provide a novel Serial No. 91,861 pflocess fwtlkllichhwill permit the artificial gging off a oys o e a ove type to pro uce pro ucts o o y 4 Clams improved mechanical properties.

The present invention relates to a novel artificial age It is a further object to provide a novel process which hardening treatment for age hardenable aluminum alloys will increase the corrosion resistance to artificially aged which undergo spontaneous age hardening at room tem- -Mgy peratures to the detriment of subsequently performed Still another ob ect of the invention 1s to provide a artificial aging treatments. More particularly the invenpr e the heat treatment of alloys which tion relates to the treatment of aluminum alloys whose W111 Petmlt subsequent artificial g hardening to Produce principal alloying elements are magnesium and silicon in eXeellent mechanlcal properties even though such alloys approximately the stoichiometric proportion of MgzSi a stored at room temperature between the time y and wherein the proportion f Mggsi i between 05% are subjected to solution heat treatment and the artificial and 3.0% and containing no more than about 0.3% of aglng treatmentcopper and preferably not more than 0.1% of copper. It Was fohnd galloys even When quenched These Al-Mg-Si alloys may also contain silicon in excess from a solution heat treatment in water at 20 C. and. of the stoichiometric proportion MgzSi, manganese in e labesatory conditions, immediately subjected tn amounts up to 1.5% and not more than a total f 1.0% artificial aging at elevated temperatures do not achieve of other alloying element h as for l copper as favorable mechanical properties as those achieved by less than 0.3% preferably less than 0.1%, Zn less than h Process In accoi'danee With the inventiohp 0.5%, Cr less than 0.5% and Fe less than 0.5%. Alloys trons p eemmerelal scale it is not practically p of the following composition are especially suitable for slhle t0 subleet e alloys t0 artificial aging at elevated treatment in accordance with the invention; Mg 0.3% P as qnlekly as is Possible in Operations 011 a to 15% preferably 07% t 10%, Si 01% t 15% laboratory scale and usually at least above five minutes preferably 0.7% to 1.2%, Mn less than 1.5% preferably Intervene between t quenching and the artificial aging between 0.6% t 10% C l h 03% preferably l In accordance with the invention it has been discovered h 01%, Z l h 03%, C l h 03%, i that substantial advantages are obtained when Al-Mg-Sl' less than 0.5% and the remainder substantially aluminum. alloys a q e from a uti n heat treatment in a Alloys of thi ty are ll bj t d to a l quenching medlum which is maintained at the tempera- 'tion heat treatment at 520 to 570 C. and are then tine required for the artificial aging of the alloys namely quenched and subsequently aged either at room tembetween ahd p y between perature or artificially aged at temperatures between 120 a and are maintained at such temperature and 200 0. preferably 140 to 160 c. to improve their untll deslretl Properties of the alley are produced. mechanical properties. When such alloys are aged at The tlme Q WhlQh the alloy is maintained depends uP011 room temperatures the following average mechanical 40 h prepertles which are sought in the y- The artifiproperties are obtained in the alloys: yield strength elal aglng y be Completed Without Permitting the article (0.2% offset) 10 to 15 kg./mm. tensile strength 20 to treated to cool down to room temperature but in some 26 kg./mm. elongation (Delta 10) 20% to 12% d a instances it has been found desirable to interrupt the Brinell hardness of 50 to 60 kg./mm. When said artlficlal aglng treatment to Permit other Operations to alloys are artificially aged at elevated temperatures the he perfehhen before t final artificial aging takes P following average values are obtained for their mechanin the latter s n It has been found Preferable to 0011- cal properties: yield strength 18 to 28 kg./mm. tensile tlnne the PfehmlharY eltifieial aging until the article strength 28 to 32 kg./mm. elongation 14% t 10% d possesses a hardness which does not increase during the a Brinell hardness of 70 to 95 kg./mm. (measured with Perled thehftlete s e at room teInPelatnfe between 2.5 mm. ball with a load of 32.5 kg. for thirty seconds). r r y artlfielal aging treatment n the final It has been known for some time that when Al-Mg-Si al'tlfielal g treatmentalloys which are poor in copper are permitted to age at The follolfvlng example illustrates the advantages of room temperatures for substantial periods of time before the Process P accordance h t tion- A series artificial aging at elevated temperatures, the mechanical of test speclnlens of an alloy composed of properties achieved are not as good as when the arti- (179% 079% M -S e and th fi i l aging i commenced i di l fte the alloys 0 remamder Al were sub ected to a solution heat treatment h b h i property f the at 550: 5 C. for one hour. One series of the specialloys has lead to problems in their heat treatment, for s Was h quenched in Water at room temperature it is not always desirable or practical to subject the and 1mmed1ately l artificial aging in an a products to an artificial aging immediately after quench- Oven at Portlon for eight hours and the ing. For exam l if t samples of an ll composed other for fifteen hours. The other series of specimens f 002% c 079% M 079% 93% was sub ected t o-th e same solution heat treatment but Fe and its remainder A1 are subjected to a solution heat was q e 011 a temperature f 150 C. and treatment at 550 C. for about 1 hour and quenched in Placed thesame alf Ovens for aging as $0011 as y water at 20 C. and a portion of su h t t samp1es i reached equilibrium temperatures, one portion also was 1mmed1ately subjected to artificial aging in an air oven aged for eight hours and h r or 15 hours. The at 150 to 155 C. for eight hours and the other portion following chart indicates the differences in the mechanical is permitted to remain at room temperature for an hour properties obtained in the two series of tests. In the before being sub ected to the same artificial aging treatchart the designation NW indicates the series quenched ment the following properties are obtainedinthe samples: in water and the designation IW indicates the series quenched in oil. sti eii in s r ih Ek'mga' k rn rn.=' k jiu m pgi' dilt Yield Strength, g g hhgp gg, LengtholArtificlal kgJmm. percent I Artilgiclally a s immediately Aging ritn r ifi igati"sonata;- NW 1W NW 1W NW quenching 26. 4 33. 5 15. 8

shout-s 34. 25 35.7 37. 25 38.3 10

Room temperature storage before artificial aging has 15 mm 1 the same detrimental effect upon all alloys of the These values show the improvement obtained by the IX'OCBSS in accordance with the invention when compared vith normally used processes carried out under the most avorable conditions, which conditions, however, cannot I6 practically achieved in commercial scale operations.

A further series of tests were carried out with eight .pecimens of the same alloy which had been subjected o a solution heat treatment. In each series of the tests ll'le specimen was quenched in water at room temperaure and after an intervening period of exactly ten min ltes placed in an air oven and heated to 150 C. and .rtificially aged therein for fifteen hours and the other pecimen was quenched in oil at 150 C. and immedi- .tely placed in the air oven and aged for fifteen hours. lpecimens 1, 3, and 7 were those quenched in water ind specimens 2, 4, 6 and 8 were those quenched in oil.

It will be seen from a comparison of the latter chart Iith the former chart that the values obtained when there i a 10 minute delay between the quenching in Water and 1e artificial aging are only slightly different from the alues obtained when the specimen is immediately sub- :cted to age hardening after quenching in water. It may, owever, be clearly seen that the process in accordance Iith the invention produces a recognizable improvement 1 the properties of the alloy treated. The average values f the properties obtained for the tensile strength and ield strength with the process in accordance with the avention are not only considerably better but the variaions are in a considerably narrower range.

It is furthermore interesting that the progress of the ardening obtained with artificial aging in accordance r ith the present invention substantially parallel that obained when the alloys are quenched to room tempera- Jre and immediately artificially aged so that for given eriods of artificial aging substantially the same hardesses are obtained.

Corrosion tests have also shown that the corrosion esistance of alloys artificially aged in accordance with me invention is considerably greater than when prior art rocesses are employed. For example, corrosion tests Iere made with test specimens of an Al-Mg-Si alloy of 1c type specified, one series being quenched in water at com temperature and immediately artificially aged at 50 C. for various periods and the other series being uenched in oil at 150 C. and artificially aged by being iaintained at such temperature for varying periods of me.

The aging periods for each series were selected as our hours, eight hours, sixteen hours and seventy two ours.

The corrosion test consisted of immersing suitably preared specimens in an aqueous half normal NaCl soluon containing Vz% of concentrated HCl and measuring 1e amount of hydrogen evolved. After three hours ealtmgnt the following volumes of hydrogen were vo ve While the above examples refer to treatments in which the total desired artificial aging is obtained in one uninterrupted aging treatment in conjunction with the quenching from a solution heat treatment, the process in accordance with the invention may be modified to permit final artificial aging to the desired properties some time after the quenching when for practical reasons, such as, lack of aging oven capacity in the plant where the alloys are subjected to solution heat treatment and quenching or where it is desired to subject the alloys to treatments which are not practical before the solution heat treatment nor after the artificial aging treatment.

It has been found in accordance with the invention that the detrimental effects of room temperature storage of Al-Mg-Si alloys of the type described between quenching and artificial aging may be substantially avoided if such alloys are quenched only to the artificial aging temperature, that is between and 200 C. and are maintained at such temperature until the artificial aging eifected thereby provides a hardness which is greater than that which would be obtained directly after quenching such alloy in water at room temperature and the final aging takes place before its hardness increases because of room temperature storage. Preferably the preliminary artificial aging treatment in accordance with the invention is of such duration that the hardness of the alloy obtained does not substantially exceed that which would ultimately be obtained if the alloy were quenched to room temperature and aged at room temperature. The preliminary aging is carried out to such ultimate hardness in such cases where the preliminarily aged alloy is to be furnished to others for fabrication and final artificial aging as the period of room temperature storage in such instances is uncertain.

While the preliminary artificial aging treatment should be long enough that the hardness of the alloy does not substantially increase during room temperature storage before it is given the final artificial aging treatment in order to secure the optimum mechanical values it has been discovered that substantially improved mechanical values may be obtained in accordance with the invention even though the preliminary aging treatment is insufficient to maintain a constant hardness for the alloy during its entire storage at room temperatures as long as the hardness of such preliminarily artificially aged alloy is still below that which would be obtained in the alloy if it were stored at room temperature for an equivalent period of time after quenching at room temperature.

For example, a test sample of an Al-Mg-Si of the type described composed of 0.02% Cu, 0.66% Mg, 0.72% Mn, 0.93% Si, 0.22% Fe and the remainder Al were solution heat treated at 550 C. for 1 /4 hours. Five of such samples were quenched in water at room temperatures and four other test samples were quenched in oil at C., one of such samples being held in such quenching oil for 1 minute and the three others for 10 minutes, 15 minutes, and 20 minutes respectively.

One of the test samples which was quenched in water originally had a Brinell hardness of about 54 kg./mm. which increased to 58 kg./mm. after 1 hour room temperature storage, to 70 kg./mm. after 15 /2 hours storage, to 74 kg./mm. after 41 hours storage and to 78 kgJmrn. after 84 hours storage.

The test sample which was quenched in oil and preliminarily aged for 1 minute had an original hardness of 41 kg./mm. which increased to 53 kg./mrn. after 1 hour storage at room temperature.

The test sample which was preliminarily aged for 10 minutes had an original hardness of 58 kg./mm. which did not increase until after about 9 hours storage at room temperature and increased to about 61 kg./mm. after 15 /2 hours storage at room temperature.

The test sample which was preliminarily aged for 15 minutes had an original hardness of about 65 kg./mm. which did not increase until after about 40 hours storage at room temperatures and had a hardness of about 65 kg./mm. after 41 hours storage at room temperature.

The test sample which was preliminarily aged for 20 minutes had an original hardness of about 70 kg./mm. which did not increase during storage at room temperatures for 84 hours.

The four other test samples which were quenched in water were artificially aged at various periods of time after the quenching and the hardness obtained compared with the test samples which had been preliminarily aged in accordance with the invention. The artificial aging treatment was for 15 hours at 152 C. The following chart shows the Brinell hardness obtained.

Storage at Room Temperature inHours Treatment Quenchedinwater 20C 114.5 95.5 89.0 88.0 Quenchedinoil150 0. for 1 minute. 100 Quenched in oil 150 C. for

minutes 110 Quenched in oil 150 0. for

minutes 116 Quenched in oil 150 C. for minutes 114 This chart clearly shows the improvements obtained in accordance with the invention when solution heat treated alloys must be stored at room temperatures before the artificial aging treatment. Even those samples which were subjected to artificial aging after a period of storage at room temperatures sufiiciently long that the hardness of the sample increased during such storage shOW a considerably greater hardness than those which were stored after quenching to room temperature.

The test sample shown in the above chart which was quenched in water and immediately artificially aged possesses a tensile strength of 36.2 kg./mm. and a yield strength of 31.0 kg./mm. whereas the sample which was quenched in water and stored at room temperature for one hour before artificial aging only had a tensile strength of 33.3 kg./mm. and a yield strength of 22.0 kg./mm. and the water quenched sample which was stored for 84 hours before artificial aging had a tensile strength of 32.6 kg./mm. and a yield strength of 20.0 kg./mm. In comparison the test sample which had been preliminarily aged for 20 minutes in accordance with the invention and which received its final artificial aging 84 hours after such preliminary aging had a tensile strength of 38.6 kg./mm. and a yield strength of 31.0 kg./mm. In other words the tensile strength obtained was 107% of that obtained in the water quenched sample which was artificially aged immediately after quenching and the yield strength equal to that obtained in the sample which was aged artificially immediately after quenching. The decided advantage of the process in accordance with the invention is clearly apparent for mechanical values may be obtained in alloys which are artificially aged long after the solution heat treatment and quenching operations which are identical with or superior to those obtained when the alloys are quenched in water and immediately artificially aged under laboratory conditions which are almost impossible to fulfill in operations upon a commercial scale.

The length of time required on preliminary artificial aging treatment in accordance with the invention depends upon the time the preliminarily aged alloy is to be stored at room temperature before the final artificial aging and also upon the temperature used for the preliminary aging, the higher the temperature the less time will be required.

The advantages of the preliminary artificial aging treatment are also apparent from the following tests which were carried out with specimens of an alloy compost of 0.79% Mg, 0.79% Mn, 0.93% Si, 0.49% Fe and 0.02% Cu. The specimens were all given a solution heat treatment for one and one-half hours at 550 C. Specimens of such alloy which were quenched to room temperature were found to possess a Brinell hardness of 45 kg./mm. which increased to about 67 kg./mm. after storage at room temperature for 21 days. Specimens that were quenched in oil at 180 C. and maintained in such oil for five minutes had an initial Brinell hardness of about 57 l g./mm. which increased to about 67 kg./mm. after 21 days storage. Specimens which were quenched in oil at 180 C. and maintained at such temperature for 10 minutes had an initial Brinell hardness of about 67 kg./mm. which did not substantially increase during 21 days storage. The following table gives a comparison of the tensile strengths and yield strengths obtained in the test specimens which were treated in various ways after the solution heat treatment.

Yield Treatment after Tensile Strength Specimen Treatment utter solution heat Strength, 0.2% Number treatment 21 days kgJmm." onset,

kg./mm 2 H 0 at 20 0-... 31.7 25.4 -do 2 hrs. at 180 C 32.9 26.1 Oil at 180 0., 28. 2 17. 9

5 minutes. d0 2 hrs. at 180 C 33. 4 28.0 Oil at 180 0., 28.9 21. 0

10 minutes. do .n- 2 hrs. at 180 0-... 34.6 33.6 Oil at 180 0., 35. 9 34. 2

2 hours.

As may be seen from this table, preliminary treatment for 5 minutes at 180 C. is insufficient to produce the optimum results, for the yield strength obtained after the subsequent artificial aging is substantially below those obtainable by artificial aging immediately after quenching in oil at 180 C. The preliminary treatment for 10 minutes at 180 C. on the other hand renders it possible to obtain both tensile strengths and yield strengths approaching those obtained with an artificial aging immediately after quenching at 180 C.

The preliminary aging treatment in accordance with the invention not only has the advantage that the preliminarily aged alloys may be stored at room temperatures between a solution heat treatment and the artificial aging thereof without substantially affecting the mechanical values obtainable by artificial aging but it also proviiles other advantages in the processing of Al-Mg-Si a oys.

Such preliminary aging treatment may, for example, be used for articles which are to be anodized and coated with an artificial resin containing lacquer which must be burned in. In this instance the anodizing and coating with the lacquer may be carried out after the preliminary aging treatment and the articles may then be subjected to the final artificial aging treatment at a temperature and period suitable for burning in the lacquer. As a consequence, it is not only possible to reduce the heat requirements for producing the finished product and also to avoid possible reduction in the mechanical properties which may occur when a lacquer coating is burned in on a fully artificially aged alloy.

Also, the preliminary aging treatment in accordance with the invention provides advantages when the article is to be formed by one or two draws after the solution heat treatment. The yield strength of the alloy after preliminary aging treatment is substantially less than that of the alloy which is aged at room temperatures and consequently less power is required for the forming operations. If two drawing operations are to succeed the solution heat treatment it has been found desirable to combine the first draw with the preliminary aging treatment. This may be easily done by taking the article to be drawn directly from the solution heat treatment and drawing it with a punch and die which are cooled to the extent that the article is quenched to the artificial aging temperature of, for example, C. while it is drawn and transferring it to an oven to complete the preliminary aging treatment and then after such preliminary aging treatment it may be subjected to the second drawing operation before it is subjected to the final artificial aging treatment. Obviously it is also possible to combine a drawing operation with the process in accordance with the invention wherein the article is quenched to the arti ficial aging temperature and is maintained at such temperature for the full desired artificial aging treatment.

The preliminary aging treatment in accordance with the invention is especially well suited for the production of semifinished products for persons who do not have the necessary equipment to carry out a solution heat treatment. For example, Al-Mg-Si alloys containing Mg and Si in the quantities specified which are prepared from aluminum of the highest purity and contain other elements only as traces are admirably suited for dental work because of their corrosion resistance and lightness. However, in fully artificially age hardened condition they are not easily formed and therefore to obtain the most desirable strengths the dentist would have to subject his products to a solution heat treatment at his ofiice. This requires more complicated apparatus than a dentist normally has at his disposal and consequently his use of such alloys is comparatively limited. Alloys preliminarily aged in accordance with the invention may, on the other hand, easily be used by the dentist for the necessary final artificial aging may easily be carried out with simple equipment such as, for example, an oil bath heated to 150 to 180 C.

The process in accordance with the invention wherein the Al-Mg-Si alloy is quenched to artificialage hardening temperatures and is maintained at such temperatures is also especially adapted for the treatment of Al-Mg-Si rivets. If the rivets are maintained at artificial aging temperatures for only a very short period of time of the order of 1 minute it is possible to produce rivets which have a lower hardness than rivets which are quenched in water at room temperature and which will have a lower hardness than the water quenched rivets for some time. This renders it possible to drive such rivets using less power than for water quenched rivets during such time. Nevertheless, such driven rivets will age at room temperatures and ultimately will have the same strength and hardness as water quenched rivets.

If the rivets are preliminarily artificially aged at the quenching temperature until they possess a hardness over that of rivets immediately after quenching in water such preliminarily aged rivets will possess a lower hardness than the water quenched rivets after a certain period of time. narily aged to a hardness equal to that of water quenched rivets which have been stored at room temperature for five hours will maintain the same hardness for about 40 hours room temperature storage and consequently will have a much lower hardness than the water quenched rivets during the period of storage between 5 and 40 hours after the quenching. Such rivets may be driven and be permitted to age naturally at room temperatures to the same hardness and strength as the water quenched rivets. If the parts which are riveted also have been subjected to a similar preliminary artificial aging treatment, the riveted article may be subjected to a final artificial aging treatment which, if it is carried out before the rivets and the parts riveted begin to increase in hardness, will produce an optimum hardness and strength in the riveted article.

While the process in accordance with the invention has been described with particular reference to AlMg-Si alloys containing only relatively minor amounts of Cu and preferably less than 0.1%, improved results may also be obtained with other high strength artificially ageable aluminum alloys which age harden at room temperatures to the detriment of the properties obtainable with subsequent artificial age hardening. The process in accordance with the invention is, however, especially For example, in some instances rivets prelimiapplicable to Al-Mg-Si alloys which contain quantities of Cu to which are insufficient to suppress the detrimental effects of aging at room temperature. While the process in accordance with the invention produces beneficial results in Al-Mg-Si alloys containing up to 0.3% Cu, such as for example 618, such alloys as contain less than 0.1% Cu, such as A518 and 538, are especially adapted for the process.

I claim:

1. A process for the artificial aging of an Al-Mg-Si alloy which spontaneously ages at room temperature after quenching to room temperature from a solution heat treatment to the detriment of values obtainable upon subsequent artificial aging which comprises quenching said alloy from a solution heat treatment to an artificial aging temperature between C. and 200 C., maintaining such alloy at such temperature until it has reached a hardness which is greater than that which would be obtained after immediately quenching said alloy in water to room temperature, storing such preliminarily aged alloy at room temperature and subjecting it to an artificial aging treatment at elevated temperatures before the storage at room temperatures causes an increase in the hardness of the alloy.

2. A process for the artificial aging of an Al-Mg-Si alloy containing 0.7%1.0% of Mg, 0.7%-l.2% of Si, and 0.6%-1.0% of Mn, and containing less than 0.1% of Cu which comprises quenching said alloy from a solution heat treatment to an artificial aging temperature between C. and 180 C., maintaining such alloy at such temperature until it has reached a hardness which is greater than that which would be obtained after immediately quenching said alloy in water to room temperature, storing such preliminarily aged alloy at room temperature and subjecting it to an artificial aging treatment at elevated temperatures before the storage at room teiinperatures causes an increase in the hardness of the a oy.

3. A process in accordance with claim 1, in which said Al-Mg-Si alloy contains less than 0.1% of Cu.

4. A process in accordance with claim 1, in which said preliminary artificial aging treatment is discontinued before the hardness produced thereby in the alloy is greater than the hardness which would be obtained by quenching said alloy to room temperature from a solution heat treatment and aging such alloy at room temperature.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,472,739 Archer et al. Oct. 23, 1923 2,239,744 Smith et al. Apr. 29, 1941 

1. A PROCESS FOR THE ARTIFICIAL AGING OF AN AL-MG-SI ALLOY WHICH SPONTANEOUSLY AGES AT ROOM TEMPERATURE AFTER QUENCHING TO ROOM TEMPERATURE FROM A SOLUTION HEAT TREATMENT TO THE DETRIMENT OF VALUES OBTAINED UPON SUBSEQUENT ARTIFICAL AGING WHICH COMPRISES QUENCHING SAID ALLOY FROM A SOLUTION HEAT TREATMENT TO AN ARTIFICAL AGING TEMPERATURE BETWEEN 120* C. AND 200* C., MAINTAINING SUCH ALLOY AT SUCH TEMPERATURE UNTIL IT HAS REACHED A HARDNESS WHICH IS GREATER THAN THAT WHICH WOULD BE OBTAINED AFTER IMMEDIATELY QUENCHING SAID ALLOY IN WATER TO ROOM TEMPERATURE, STORING SUCH PRELINIMARILY AGED ALLOY AT ROOM TEMPERATURE AND SUBJECTING IT TO AN ARTIFICAL AGING TREATMENT AT ELEVATED TEMPERATURES BEFORE THE STORAGE AT ROOM TEMPERATURES CAUSES AN INCREASE IN THE HARDNESS OF THE ALLOY. 